First stage chained straight off the bitstream rather than a fixture: parse the
container, take the SOZ substream's bytes, and run the byte-exact mANS `fac`
(factorized) decoder over it with the normative per-channel CDFs. Symbols are offset
by 31 (the mid-point of the 63-symbol alphabet) to give signed z.
SOZ carries both planes back to back — luma (160 ch) then chroma (96 ch), 16 samples
per channel — decoded by ONE MansDecoder whose ANS state threads across the two calls.
Validated on test256.bin: z[y] 2560/2560 exact (75 non-zero, range [-3,4]) and
z[uv] 1536/1536 exact, both equal to the reference z that decode_hyper_scale consumes.
This joins the two halves of the decoder: the container/entropy path now reproduces,
from raw bytes, the exact tensor the neural scale path was validated against.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
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